The present invention relates to a method for lapping a thin disc-like or plate-like material and an apparatus used therefor or, more particularly, to a method for lapping a thin material using a lapping machine provided with an upper and a lower lapping surface plates rotating relative to each other with the work piece sandwiched therebetween and an improved lapping machine for practicing the method with which in-process determination and control of the thickness of the work piece under lapping can be readily performed.
It is a usual process for precision-lapping of a thin disc-like or plate-like material, e.g. semiconductor silicon wafers, that the work piece is sandwiched as supported by a carrier between an upper lapping surface plate and a substantially parallel lower lapping surface plate of a lapping machine rotating relative to each other with continuous supply of a lapping fluid containing fine particles of an abrasive material until desired exactness of the surfaces of the work piece is obtained.
With the recent progress in the electronics or other fine technologies, it is sometimes required that the thickness of lapped materials is controlled with very high accuracy with an error of 1.times.10.sup.-3 mm or smaller. The most simple but reliable way for the determination of the thickness of the work pieces under lapping is so-called out-of-process methods in which the lapping machine is periodically interrupted and the work pieces under lapping are taken out of the machine to have the thickness measured by a conventional measuring means. This method is, of course, very troublesome or time-consuming and undesirable from the standpoint of working efficiency.
Accordingly, there have been made several attempts to develop a method for the in-process determination of the thickness of work pieces under lapping, in which measurement of the thickness can be carried out without interrupting the operation of the lapping machine (see, for example, Japanese Utility Model Publication 41-24476). One of the problems in these prior art methods for the in-process thickness determination is that what is measured by the method is not the thickness of the work piece under lapping itself but the distance between the surfaces of the upper and the lower lapping surface plates. Therefore, large errors are sometimes unavoidable in the thickness determination due to the wearing or other irregularities in the plate surfaces and the intervention of the abrasive particles between the work piece and the plate surfaces. These errors are, in particular, relatively large when the thickness of the work piece under lapping is small, for example, 2 mm or smaller. Thus, no satisfactory method for the in-process determination of the thickness of work pieces under lapping has yet been developed and most of the conventional lapping processes utilize a mere timer with which a predetermined lapping time is set for interrupting the operation of the lapping machine.